New 67 Bonni Conrods?

[Twinpots]

Hi all. Doing a complete engine strip on my 67 Bonneville. A little sooner than I expected due to a holed piston. 20,000 klm since a rebore to plus 20 and fitting brand new Monoblocs. Guessing it was due to spirited riding (over 5000rpm) in 38 deg heat causing too lean a mix. I normally rarely exceed 4000 rpm so this issue never showed itself previously and the plugs always looked spot on. Anyway s**t happens and now I may as well do a complete rebuild. Looks to also include new cam bushes as the timing side feel a little loose. Not I job I would have chosen.
Now, as the heading mentioned conrods I better get to the point. The rods look good but of course the little end on the left rod was exposed to combustion heat when the piston holed itself. I have read all sorts of horror stories concerning rods not taking this heat abuse too well. With a matching numbers bike I would hate to lose the crankcase should the worst happen. Done some research and have seen the variety of new rods available. The MAP steel rods look suitable at ok ish money but my query is would there be a problem balancing the crank with heavier rods? My crank is the thinner lighter weight type as fitted to the 67 Bonneville so there may not be much to play with with regarding balancing for heavier rods. Not heard good things about the quality of the LF Harris made rods which is a shame. Mine are original Triumph which may make them the better choice to stick with. Please, anyone with first hand knowledge of fitting new replacement rods to this engine give me your thoughts.
One item other owners may find interesting. My bike has done approx. 25,000klm in my possession with no known prior history. The speedo before I had it changed to klm's read 11,000 miles US. On checking the sludge trap there was almost nothing in there and the small amount that there was simply washed away with paraffin. Since getting the bike I have religiously changed the oil between 1200 klm and 1500 klm. Doing this would appear to almost completely prevent sludge build up. I had always wondered what state my sludge trap was in and now I know. Change oil a lot and I reckon the main bearings or big ends will need changing before the sludge trap causes a problem at which time you then of course include the Sludge trap. Easy peasy.
Just so you know with regards to other wear rates, the timing side main bearing was a tiny bit notchy and the drive side was good. Big ends (standard size) showed little wear and no damage. Hope this gives others a better idea of wear on these engines. Although none were leaking all the oil seals were rock hard and split so easily when touched. I would say this is the weak spot on these engines if they are the original factory fitted. Yet to look into which oil seal maker is the best for replacements. Any suggestions please let me know. Cheers. (Photo shows Concentrics before swapping to Monoblocs). There is a full sized Dinosaur in the background, can you spot it?

 

[CarlS]

I will be following this thread.

 

[grandpaul]

Get the engine open and do a thorough visual inspection BEFORE cleaning any evidence off the parts.

Get lots of pix.

Then, proceed for check every spec in the shop manual "specs" section and compare with allowable limits.

OEM classic Triumph rods are plenty robust; not sure where your concern came from, but it MIGHT be a bit over-thought...

Please report back.

 

[Twinpots]

Hi GP. Absolutely over thinking this. Lurid tales of conrods letting go (put out by conrod manufacturers?). I am not building a 8,000 rpm racing engine so its good to hear an opinion that the original rods should be fine. I do of course polish out any scratches as is good engineering practice. I have purchased a new set of rod bolts from Trojan spares here in Australia but they don't say they are forged or who made them (think I will give them a call). Everything I read recommends forged bolts such as those supplied by ARP. Back in the good old days before the internet I would have simply re used the old bolts not knowing any better. The current main bearings are RHP which it seems are the best given the new prices being asked. Are they really worth the extra over other makers? I will give the +20 bores a quick hone and by an odd quirk of serendipity I just happen to have a second brand new pair of +20 Hepolite pistons on the shelf. I never thought I would use these expecting to go up a size if it was ever required. Next paranoia is balancing the crank. Real lack of engineering outfits here that will understand these old Triumphs. Would love to have it dynamically balanced but terrified they will stuff it up. I believe Triumph went for about 75% balance but what are your thoughts to achieve the smoothest result at say 3000- 4000 rpm? This matches are speed limits here. Can I get away with static balancing as per the original Triumph method?
Anyone found the Dinosaur yet?

 

[Greyfell]

There is a full sized Dinosaur in the background, can you spot it?

Right side of the photo, in the sky, nearly straight up from the fence post...a large "bird".

 

[Lordmac]

Hi all. Doing a complete engine strip on my 67 Bonneville. A little sooner than I expected due to a holed piston. 20,000 klm since a rebore to plus 20 and fitting brand new Monoblocs. Guessing it was due to spirited riding (over 5000rpm) in 38 deg heat causing too lean a mix. I normally rarely exceed 4000 rpm so this issue never showed itself previously and the plugs always looked spot on. Anyway s**t happens and now I may as well do a complete rebuild. Looks to also include new cam bushes as the timing side feel a little loose. Not I job I would have chosen.
Now, as the heading mentioned conrods I better get to the point. The rods look good but of course the little end on the left rod was exposed to combustion heat when the piston holed itself. I have read all sorts of horror stories concerning rods not taking this heat abuse too well. With a matching numbers bike I would hate to lose the crankcase should the worst happen. Done some research and have seen the variety of new rods available. The MAP steel rods look suitable at ok ish money but my query is would there be a problem balancing the crank with heavier rods? My crank is the thinner lighter weight type as fitted to the 67 Bonneville so there may not be much to play with with regarding balancing for heavier rods. Not heard good things about the quality of the LF Harris made rods which is a shame. Mine are original Triumph which may make them the better choice to stick with. Please, anyone with first hand knowledge of fitting new replacement rods to this engine give me your thoughts.
One item other owners may find interesting. My bike has done approx. 25,000klm in my possession with no known prior history. The speedo before I had it changed to klm's read 11,000 miles US. On checking the sludge trap there was almost nothing in there and the small amount that there was simply washed away with paraffin. Since getting the bike I have religiously changed the oil between 1200 klm and 1500 klm. Doing this would appear to almost completely prevent sludge build up. I had always wondered what state my sludge trap was in and now I know. Change oil a lot and I reckon the main bearings or big ends will need changing before the sludge trap causes a problem at which time you then of course include the Sludge trap. Easy peasy.
Just so you know with regards to other wear rates, the timing side main bearing was a tiny bit notchy and the drive side was good. Big ends (standard size) showed little wear and no damage. Hope this gives others a better idea of wear on these engines. Although none were leaking all the oil seals were rock hard and split so easily when touched. I would say this is the weak spot on these engines if they are the original factory fitted. Yet to look into which oil seal maker is the best for replacements. Any suggestions please let me know. Cheers. (Photo shows Concentrics before swapping to Monoblocs). There is a full sized Dinosaur in the background, can you spot it?View attachment 57026

Nice bike. Bummer about that piston though.
I re-installed my original Triumph connecting rods (beautiful things, aren't they?) and bolts. No problems. The photo of yours does not seem to show a serious difference between the two. The heat from the melted piston should show significant discoloration or disfigurment on the little end, and close study might reveal damage - or not. but your desire to NOT have a problem is key. If they were mine, I would use the originals, given they both look and spec the same.
I had bad luck with my original-equipment timing side RHP5/6306 bearing. It went SOUTH on me after a mere six thousand miles and was replaced with a thirty dollar SKF which has held up nicely.
The only dino-saur I see in the photo is that motorcycle . . . no, wait! Yes! I see it now . . .